The use of optical data storage disks having a single data storage layer is known. Such disks include, for example, compact disks (CDs) and CD-ROMs, magneto-optical disks (M/O), and some digital video disks (DVD). More recently it has been proposed that optical data storage disks be increased in storage capacity by having multiple data storage layers, and limited use of two layers has been introduced on the market. Such multilayer disks would have a plurality of inner data storage layers and an outer protective layer for protecting the data storage layers from scratches and dust.
In such storage disk systems, a focused beam of light is reflected from a modulation at a point in a data storage layer, such as a pit, magneto optical bit, phase change bit, or dye polymer bit, and the way in which the beam is reflected determines the value of the data stored at that point.
It is also known that both the data storage layer and protective layer cause a spherical aberration to the light accessing both layers, which is different for different data layers, and which distorts the beam which accesses the data layer. Hence, lens systems which access the optical information must correct for spherical aberration in order to increase the density of data in the disks.
In the past, such lens systems have required different lenses corresponding to different thicknesses of the protective layer, and thus can not be used for varying thicknesses of the protective layer. In addition, such lens systems have required complex structures to access the multiple data layers such as more than one actuator with a corresponding motor for use on more than one movable lens. Other prior lens systems have not been able to access more than two different storage layers of the media.
All of these factors have resulted in an unduly complicated structure for the prior lens systems.